This invention relates in general to motor vehicle roll control systems and in particular to a self-centering actuator for a roll control system, and a roll control system including a self-centering actuator.
Suspension systems for a motor vehicle are known which isolate the vehicle load from irregularities in the terrain over which the vehicle travels. A semi-active suspension system, for example, normally includes a spring and a damper connected between the sprung and unsprung portions of the vehicle. Semi-active suspension systems are generally self-contained, and only react to the loads applied to them. In active suspension systems, by contrast, the reactions to the applied loads are positively supplied, typically by electronically controlled hydraulic or pneumatic actuators.
In addition to isolating the sprung portion of the vehicle from the road, it is desirable to stabilize the vehicle and resist the tendency of the sprung portion of the vehicle to tilt or roll relative to its unsprung portion when accelerating, decelerating or cornering at relatively high rates. Therefore, suspension systems have been proposed to maintain the vehicle in an essentially level position, regardless of the source of the force seeking to upset that position. For example, U.S. Pat. No. 5,630,623 to Ganzel, the disclosures of which are incorporated herein by reference, discloses a semi-active system for controlling the roll of a motor vehicle including an actuator connected between an unsprung portion of the vehicle and a sprung portion of the vehicle. The known roll control system of U.S. Pat. No. 5,630,623 not only locks and unlocks the anti-roll bars of the vehicle, but also accommodates the upward or downward deflections of any of the four wheels of the vehicle, regardless of whether the affected wheel is on the inside or outside of the turn, as will be described in more detail below.
Referring now to the drawings, FIGS. 1 and 2 show the prior art semi-active, pumpless system 10 for controlling the roll of a motor vehicle according to U.S. Pat. No. 5,630,623. The system 10 comprises a front hydraulic circuit 11a and a rear hydraulic circuit 11b. The front hydraulic circuit 11a includes a front actuator 12, first and second pressure control or pressure relief valves 14 and 16, and first and second check valves 18 and 20. The rear hydraulic circuit 11b includes a second rear actuator 21 and other components similar to those of the front hydraulic circuit 11a which will be discussed below.
Each of the wheels 22, 24, 26 and 28 of the vehicle is rotationally mounted about a substantially horizontal axis to a member such as suspension arms 30, 32, 34 and 36, respectively, which form part of an unsprung portion of the vehicle. The unsprung portion of the vehicle is in turn connected to a sprung portion of the vehicle through the actuators 12 and 21 and anti-roll or anti-sway bars 38 and 40.
Each of the actuators 12 and 21 includes a cylinder 42 and a piston 44 reciprocably disposed in the respective cylinder 42. One of the cylinder 42 or the piston 44 of each actuator 12, 21 is drivingly connected to an associated one of the anti-roll bar 38, 40 or the suspension arm 30, 32, 43, 36 while the other component of each cylinder/piston pair is drivingly connected to the associated other of the anti-roll bar 38, 40 or the suspension arm 30, 32, 43, 36. As shown in FIGS. 1 and 2, for example, the cylinder 42 of the front actuator 12 is connected to one free end of the front anti-roll bar 38, while the portion of the piston rod extending generally downwardly from the cylinder is connected to the front right suspension arm 30. Similarly, the rear anti-roll bar 40 is coupled to the cylinder 42 of the right rear actuator 21 while the piston 44 of the actuator 21 is connected to the suspension arm 32.
The actuators 12 and 21 each have a pair of ports, respectively 46, 48 and 50, 52, through which a working medium such as hydraulic fluid may be alternately provided to or evacuated from the ends of the cylinders 42 disposed on either side of the pistons 44 situated therein. As described more fully below, each of the actuators 12 and 21 serves to maintain the sprung height from the road surface of the portion of the vehicle body above its associated wheel.
The first pressure control valve 14 is a proportional relief valve, and is in communication with the first port 46 of the actuator 12 through a hydraulic line 54. The valve 14 is operated by a proportional solenoid, and has an open position and a closed position. In response to actual or anticipated loading of the actuator 12, the solenoid energizes the valve 14 toward the closed position with a force proportional to an electric signal applied thereto, which prevents flow away from the first port 46 until a predetermined pressure develops in the upper chamber of the cylinder 42 to overcome the solenoid force, as described more fully below. The first check valve 18 is situated in the hydraulic circuit in parallel with the first pressure control valve 14, and permits flow therethrough only in a direction toward the first port 46 of the actuator 12.
The second pressure control valve 16 is also a proportional relief valve, and is in communication with the second port 48 of the actuator 12 through a hydraulic line 56. The valve 16 is also controlled by a proportional solenoid, and can be moved between open and closed positions by the solenoid to prevent flow away from the second port 48 until a predetermined pressure develops in the lower chamber of the cylinder 42. The second check valve 20 is situated in the hydraulic circuit in parallel with the second pressure control valve 16 and permits flow therethrough only toward the second port 48 of the actuator 12.
The rear hydraulic circuit 11b for the rear actuator 21 is substantially identical to the hydraulic circuit 1a for the front actuator 12. Thus, a proportional pressure control valve 58 and a parallel check valve 60 are provided in communication with the first port 50 of the rear actuator 21, and another proportional pressure control valve 62 and a parallel check valve 64 are provided in communication with the second port 52 of the rear actuator 21.
In operation, an electronic control unit (ECU) 70, processes inputs from one or more wheel speed sensors 72, a lateral accelerometer 74, and a steering angle sensor 76. Given these inputs, the ECU 70 predicts the severity of an upcoming roll, and issues control commands to the solenoids of the appropriate valves 14 and 58 or 16 and 62. For example, the motor vehicle may begin a relatively high speed left hand turn. Such a turn, in absence of compensation by the system 10, would cause the unsprung portion of the vehicle to tend to roll generally clockwise about it longitudinal axis.
At the beginning of such a maneuver, sensors 72, 74 and 76 signal the instantaneous conditions to the ECU 70. The ECU 70 in turn calculates or obtains from a look up table the net pressure P that needs to be developed in the upper chambers of the cylinders 42 of one or both of the actuators 12 and 21 to counteract the vehicle roll, and energizes the solenoids of the pressure control valves 14 and 58 an amount sufficient to resist flow through those valves up to the pressure P.
To counteract anticipated vehicle roll in the opposite direction, for example as might be experienced during a right hand turn, the ECU 70 repeats this procedure and energizes the solenoids of the valves 16 and 62 to allow build up of the pressure in the lower chambers of both actuator 12 and 21. In either case, as the sensors 72, 74 and 76 indicate an instantaneous or anticipated reduction or increase in the need for counteracting vehicle roll, the ECU 70 signals the appropriate pressure control valves to correspondingly reduce or increase their pressure cut out limit.
If an unexpected load is imposed on one of the actuators, such as might occur when one wheel rolls over a bump in the road, an increased pressure is developed in one chamber of the affected actuator. For example, if the right front wheel 22 encounters a bump and deflects upwardly during a left hand turn, the piston 44 is displaced upwardly in the cylinder 42 and the pressure in the upper chamber of the actuator 12 increases. Even if the valve 14 is energized closed at this time (to resist the anticipated roll), the increased pressure overcomes the solenoid force, allowing the suspension to compress and maintain ride quality. After the wheel 22 then passes over the top of the bump and begins to move downward and the valve 14 closes again, the check valve 18 allows the piston 44 and the suspension arm 30 to fall back down without any resistance from the roll control system. This process takes a finite amount of time, during which the vehicle body will likely roll to the right to some extent, due to the left hand turn. Since fluid was displaced out of the upper chamber of the actuator 12, the piston 44 and the suspension arm 30 are unlikely to return entirely to their original positions by the time pressure in the upper chamber of the actuator 12 returns to it's pre-upset level, and the actuator 12 will remain in a slightly more compressed state than prior to the wheel hitting the bump, allowing more than desired roll to the right during the left hand turn.
Again, considering a left-hand turn, in the event that the left front wheel 28 suddenly rises while the valve 14 is energized closed, the suspension arm 36 moves upwardly and the anti-roll bar 38 above the actuator 12 moves to a new upwardly elevated position relative to the control arm 30, acting to extend in the cylinder 42, and reducing the pressure in the upper chamber of the actuator 12. If the pressure in the upper chamber drops below the pressure in pre-charged accumulator 78, the check valve 18 allows flow into the upper chamber of the actuator 12 so that there is essentially no resistance from the actuator 12 to the extension of the actuator 12. The new extended position may be near the original position or may be beyond, i.e., in a more extended position than the original position, depending on the amount of flow allowed into the upper chamber of the actuator 12.
Again, considering a left-hand turn if the anti-roll bar 38 were originally in a center or neutral position and, for example, the valve 14 is energized with the anti-roll bar 38 out of the original position, when the anti-roll bar 38 is extended into a new position, that new position may be near the original position or past, i.e., past the central or neutral position to a more compressed position than the original position. Thus, it can be said that the actuator 12 can go beyond neutral in both directions, i.e., the actuator 12 can end up in position in which the actuator 12 is either extended more or compressed more than the condition of the actuator 12 at the neutral position.
The accumulator 78 is situated in each of the front and rear hydraulic circuits in communication with the pressure control valves 14, 16 and 58, 62. By maintaining the fluid in the hydraulic circuits under a certain pressure, the accumulator 78 functions to prevent cavitation in the system 10 when the wheels of the vehicle deflect, and also acts as a reservoir to replenish any fluid lost by the system to leakage past dynamic seals. All of the valves and the accumulator for each of the front and rear hydraulic circuits 11a and 11b are packaged in units 80 and 82 mounted near the anti-roll bars 38 and 40. The front and rear hydraulic circuits 11a and 11b are kept separate so that the valves of each circuit can ride with their respective anti-roll bar, which eliminates the need for running expensive flexible hydraulic hoses from the body of the vehicle to the front rear anti-roll bar.